The present invention relates to a speech detector for determining the presence or absence of voice signals on a telephone line, particularly on time-division multiplexed telephone channels.
The rapid expansion of communication networks has increased the demand for long distance communication links involving satellites and submarine cables. In order to obtain more efficient use of expensive long distance communications links, the so-called TASI (Time Assignment Speech Interpolation) system has been put into use. Also, echo suppressors are in use to suppress the echo caused by the long distance communication channels. An essential requirement for both the TASI system and the echo suppressor is the need for a high-performance speech detector. For reliable operation the response time of speech detection to the voice signal must be as short as possible. Otherwise, initial clipping is inevitable, deteriorating the conversation quality. Therefore, the demand is imminent for a high-performance speech detector capable of extremely high speed detection and low susceptibility to noise.
Most conventional speech detector circuits are of the amplitude-comparison type which relies upon the comparison of the voice signal level with a predetermined threshold voltage reference level for detection. This type of speech detection is capable of responding fairly quickly to most voice sounds and plosives. However, its performance is generally poor for fricative consonants such as (S) and (h), whose reference level sometimes becomes as low as background noise inherent in communication channels. This is due to the fact that the lowering of the threshold reference value to enhance the sensitivity contributes to the prevention of the initial clipping but aggravates the vulnerability to noise, while the raising of the threshold value results in the slowdown of speech detection response, causing the initial clipping.
To achieve a better detection of fricative consonants, attempts have been made for the parallel use of another speech detector of the periodicity detection type combined in parallel with the above-mentioned amplitude-comparison type one. The former relies for its speech detection on the fact that a voice signal has periodicity while noise does not. However, since the fricative consonants are inherently noise-like and lack sufficient periodicity, signals representing such sounds are still difficult to detect, so far as the periodicity is strictly relied on for the detection. In addition, an ordinary periodicity-detection type speech detector involves the problem of being quite complicated.